The present invention relates to closed-loop servo motors, and more particularly to a method and system for retuning a closed-loop servo motor.
Generally speaking, a servo motor includes a motor, a feedback device, and a drive. The motor operates on direct current, and is typically hotter and smaller than other motors producing a comparable amount of torque. The feedback device is often an encoder or resolver mounted on the back of the motor, and the feedback device reports performance information such as motor position and motor speed back to the drive. The servo motor""s drive provides current to the motor, and the drive typically includes a programmable control device (e.g., a controller) which dictates the current in response to the feedback from the feedback device. The most widely used algorithm for servo motor control is the proportional-integral-derivative (PID) algorithm, according to which various programmable filter parameters are used, depending upon the particular application, to determine the current in response to the feedback.
Over time, a servo motor and/or the equipment which it actuates may incur wear and tear, or otherwise be affected by aging, and therefore it is well known that the performance and lifespan of a servo motor can be increased by tuning the servo motor, for example by adjusting the PID filter parameters. Various tuning systems for servo motors have been devised in the past, and those systems often require manual retuning which can be very costly and time-consuming. It is very difficult or impossible for manual retuning to measure performance using actual motor profiles with actual mechanical loading, especially if actual loading is based on paper flow through a system.
Automatic self-tuning methods for electrical motors are also known, the method of Taylor et al. (U.S. Pat. No. 5,834,918) being an example. The automatic, related-art self-tuning methods such as Taylor suffer from implementation complexity. They typically use mathematical modeling in order to anticipate how the motor must be tuned, and therefore use formulas to determine various tuning constants based upon motor measurements, instead of relying upon predesigned sets of tuning constants determined in a laboratory. The related art thus is unnecessarily complex and time-consuming, and furthermore fails to take advantage of measurement and diagnostic equipment available in the laboratory of the manufacturer or distributor.
The primary objective of this invention is to provide a servo motor tuning method appropriate for a specific type of motor application, such as paper handling or processing. Because the application is known in advance, it is possible to use laboratory testing in order to determine how a specific type of servo motor will behave over its lifetime, and to determine by manual retuning how that servo motor model will have to be automatically retuned. The present invention therefore dispenses with complex mathematical modeling installed in the servo motor tuning software, and instead takes advantage of laboratory testing on a particular type of servo motor. This method has the further advantage that diagnostic equipment available to the manufacturer can be used to test and tune the servo motor, whereas such equipment would not ordinarily be available to end users. The present invention efficiently measures performance using actual motor profiles with actual mechanical loading, for example with actual loading based on paper flow through a system, in order to produce data which is then inserted into the programming for the servo motor retuning software.
Accordingly, the present invention includes a method for adaptively retuning a closed-loop servo motor that is operating within normal limits, in order to automatically retune the servo motor and thereby improve performance of the servo motor. The method comprises the sequential steps of: assigning a first active set of configurable tuning constants to the servo motor in order to operate the servo motor, periodically measuring actual performance of the servo motor while the servo motor is operating within the normal limits, determining if the actual performance of the servo motor differs from a servo motor specification performance by an amount representing a performance deficit that is outside a retuning threshold but within the normal limits for the servo motor, maintaining the active set of configurable tuning constants if the performance deficit is within the retuning threshold, and deactivating the active set of configurable tuning constants and activating a replacement active set of configurable tuning constants for the servo motor if the performance deficit exceeds the retuning threshold but is within the normal limits. According to this method, all sets of configurable tuning constants are selected from a finite group of discrete predesigned sets of tuning constants, and the replacement active set of configurable tuning constants is incrementally different from the active set which it replaced. Actual performance of the servo motor is remeasured, and this process is repeated if the performance deficit remains outside the retuning threshold but within the normal limits.
This method is implemented by a combination of hardware and software, and the predesigned sets of tuning constants will have been derived and stored in response to laboratory testing of the servo motor. This laboratory testing will typically be performed in a laboratory, workshop, or office of the manufacturer or distributor of the servo motor or the equipment which will be operated by the servo motor. Furthermore, the predesigned sets of tuning constants are derived not just in response to laboratory testing of the servo motor, but also in response to testing of the particular type of equipment operated by the servo motor, and that equipment is substantially the same as the equipment which will be operated by the servo motor when the servo motor is ultimately used by the end user. The predesigned sets of tuning constants may advantageously include programmable filter parameters of the PID algorithm.
The present invention may be less useful to servo motor manufacturers who have no way of knowing the precise type of application to which the motor will be put, and therefore accurate laboratory simulations will not be an ideal way of determining the tuning constants. However, for a manufacturer of a particular type of equipment (e.g., paper processing equipment) in which servo motors are installed, the present invention can be an efficient way of predetermining the sets of tuning constants that will be successively employed after this type of equipment is sold to an end user.